The subject invention relates to small spot ellipsometer useful for making measurements on semiconductor wafers. The ellipsometer is preferably of the single wavelength type capable of making highly accurate measurements of very thin films formed on substrates.
Ellipsometric techniques for analyzing samples have been used for quite some time. In a basic system, a probe beam having a known polarization state is directed to interact with the sample. An analyzer is provided for determining the change in the polarization state of the beam induced by its interaction with the sample. By analyzing the change in polarization state, characterizations about the sample can be evaluated. At the present time, these systems are commonly used to analyze characteristics such as thickness, refractive index and extinction coefficient of very thin films formed on substrates.
Some prior art ellipsometers utilize lasers to generate a narrow wavelength band of light to define the probe beam. Other ellipsometers utilize polychromatic or white light sources to generate the probe beam. The latter type systems can produce multiple measurements over a broad range of wavelengths. Such multiple measurements can be very useful. One drawback with a broadband system, however, is that since the probe beam is generated from a non-coherent light source, the size of the region on the sample which can be imaged is typically larger than can be achieved with a probe beam from a laser. Today, semiconductor manufacturers often want to make measurements in extremely small regions and therefore laser sources having a coherent output are used to obtain a smaller focused probe beam spot. In addition, gas discharge lasers are often used since they produce a very stable wavelength output which enhances the repeatability of the measurements.
The subject invention is directed to an ellipsometer which utilizes a laser for generating a narrow-band of radiation for use as the probe beam. The assignee herein has developed such a narrow-band ellipsometer for commercial use. Such an ellipsometer is disclosed in U.S. Pat. No. 5,798,837, incorporated herein by reference.
The current commercial embodiment of the assignee""s single wavelength ellipsometer includes a helium-neon gas discharge laser generating a probe beam at 633 nm. The beam is focused onto the sample using a fused silica lens having a numerical aperture of 0.035 and a focal length in excess of 100 mm. This arrangement produced an elliptical probe beam spot on the sample having a diameter, on the order of 20xc3x9740 microns. (Unless stated differently, the diameter referred to in this specification and claims is the 1/e2 diameter.) Given that the intensity distribution of the beam has gaussian characteristics (i.e. is larger than the 1/e2 diameter), this beam spot size was suitable for measurements on features (pads) on the wafers of about 100xc3x97100 microns.
Recently, semiconductor manufacturers are seeking instruments that can obtain measurements on pads of only 50xc3x9750 microns. To achieve this goal, a lens system must be developed with a larger numerical aperture and a shorter focal length in order to reduce the spot size in the longer axis down below 25 microns and preferably less than 20 microns.
An increase in the numerical aperture and a reduction of the focal length of a lens requires increasing the curvature of the lens. As the lens curvature increases, spherical aberrations increase, reducing the ability of the lens to properly focus the light.
There are a few relatively common methods for addressing the problem of spherical aberration. The first is to use a compound lens systems where a second lens is used to correct the spherical aberrations induced by the first lens. This approach can lead to certain problems. More particularly, if more than one lens is used, the spacing between those two lenses must be controlled very accurately as any variation due to temperature or other factors will induce errors in the measurement. In order to achieve the necessary level of stability, relatively rigid mounts are necessary which can create stress birefringence in the lens as discussed below.
Another approach is to use aspheric (non-spherical) focusing surfaces on the lens. Using sophisticated computer modeling, aspheric surfaces can be designed which will correct for any spherical aberrations created by the lens. However, these surfaces can become quite complex. In practice, it is quite difficult to machine these complex surfaces accurately and repeatably. Any inaccuracies in the lens surface can cause unacceptable diffraction problems.
Another approach to obtaining a small spot size on the sample is to place a small aperture in the path of the probe beam. By using an aperture, the requirements for lenses are relaxed. However, this approach is undesirable since it is extremely sensitive to alignment and vibration errors.
Accordingly, it would be desirable to create an ellipsometer that can produced a small spot that does not have the problems associated with the above-described solutions.
In accordance with the subject invention, an ellipsometer is provided which includes a laser for generating a narrow-band, preferably single wavelength probe beam. The probe beam is directed to reflect off the sample at a non-normal angle of incidence. The polarization state of the reflected probe beam is analyzed to determine any changes thereto induced by the interaction with the sample. As discussed below, various known combinations of polarizers and retarders can be used to control and analyze the polarization state of the probe beam.
In accordance with the subject invention, an improved lens system is provided to obtain the small spot size necessary for measurements on pads having 50xc3x9750 micron dimensions. More specifically, the lens system includes a lens having at least one spherical surface. In addition, the lens is formed from a material whose index of refraction varies along the optical axis thereof in a manner to reduce spherical aberrations.
Lenses formed from materials whose index of refraction varies in this manner are commercially available from LightPath and sold under the name Gradium(copyright) glass. Various types of Gradium glasses are available, each of which have different variations in the index of refraction. The selection of the lens material is dependent upon the application, including the desired numerical aperture and focal length.
Unfortunately, the lenses available from LightPath have a relatively high coefficient of thermal expansion, on the order of 8xc3x9710xe2x88x926/K or roughly ten times greater than fused silica lenses (coefficient on the order of 6xc3x9710xe2x88x927/K). Lenses having a high coefficient of thermal expansion can give rise to problems since they are typically held in fixed lens mounts. More specifically, as the ambient temperature varies, the lens expands and contracts against the lens mount. This variation in size varies the stress placed on the lens. Stress in the lens creates birefringence which directly effects the polarization of the beam. Thus, temperature variations result in variations in the birefringence of the lens which can significantly reduce the accuracy of the measurement.
This problem was overcome by using a low stress lens mount. This mount, which was previously developed and sold by the assignee for its single wavelength ellipsometer, includes three resilient O-rings configured to support the front, back and radially outer edge of the lens. It was determined that this low stress lens mount operated successfully to minimize stress birefringence in the Gradium lens material lens allowing this type of material to be used.
In experiments, using a lens with an effective numerical aperture of 0.1 and a focal length of 40 mm, an elliptical spot size having dimensions of only 8xc3x9716 microns was achieved.
Further details of the subject invention will be discussed with reference to the appended drawings in which: